1. Field of the Invention
The invention is in the field of micro-scale photovoltaic devices, more particularly photoelectrochemical/photoelectrosynthetic devices, processes and systems for utilizing solar energy to drive chemical reactions.
2. Review of Technology
There is great interest in renewable energy generation in order to replace conventional fossil fuels. This includes utilizing solar, wind and biomass for producing fuels. Solar energy is particularly interesting because it is a fundamental renewable energy source that can be used to continuously, noiselessly, and passively generate fuels once an infrastructure for fuel production has been developed. Photovoltaic devices consist of semiconductor materials that are capable of capturing photons from solar irradiation and converting them into electrical energy (i.e., a current having an electrical potential).
Solar cells are effectively used to provide power for satellites in space operations and stand alone appliances that are not connected to the electrical grid. However, connecting solar cells to a grid is far more complicated because the direct current power produced by solar cells must be converted to AC power according to the power transmission system into which they are to be integrated. Moreover, solar cells require complex and relatively expensive wiring to interconnect a system of solar cells together to provide electrical power to be input into the grid. Corrosion, heat stress and internal short-circuits can reduce or eliminate the ability of a solar cell to generate power. In the case of solar panels, failure of a subsection may cause catastrophic and irreparable failure and require replacement of the entire panel.
Solar energy can also be used in photoelectrosynthetic processes to produce desired products through photo-oxidation and photo-reduction of chemical components in a feedstock. For example, U.S. Pat. No. 4,263,110 to Meyerand et al. discloses the use of semiconductor “platelets” suspended in a reactor vessel containing aqueous hydrobromic acid to produce hydrogen gas and bromine as products (see FIGS. 1A and 1E of this disclosure). The semiconductor “platelet” 100 shown in FIG. 1A includes an n-doped semiconductor material 101 and a p-doped semiconductor material 102. An edgewise insulation material 103 (e.g., conventional epoxy resin) is shown broken away to expose ohmic contacts 104.
U.S. Pat. No. 4,094,751 to Nozik discloses Schottky-type and p-n junction type photochemical diodes. As shown in FIG. 1B, a cross-section of an exemplary Schottky diode 110 is shown suspended in a reaction matrix 118 of a reactor 111 and includes an appropriately doped semiconductor 112 (n- or p-type), an ohmic contact 113 adjacent to the semiconductor, and a metallic contact or layer 114 adjacent to the ohmic contact 113. The Schottky diode 110 further includes a semiconductor/matrix interface 115 and a metal/matrix interface 117. Absorption of light energy 116 by the semiconductor layer 112 creates electrons and holes (not shown). For n-type semiconductors, electrons move across the ohmic contact 113 to the metallic layer 114, where they are injected through the metal/matrix interface 117 into the reactant matrix 118 to drive a reduction reaction (such as H2 evolution). Holes are injected through the semiconductor/matrix interface 115 into the reactant matrix to promote an oxidation reaction (such as the formation of O2 or H2O2). The charge flows are reversed for p-type semiconductors.
FIGS. 1C and 1D show exemplary p-n junction type photochemical diodes disclosed in Nozik. As shown in cross-section in FIG. 1C, an exemplary side-by-side p-n junction type photochemical diode 120 is configured to absorb incident light 121 on one side and includes a p-type semiconductor 123, which is provided with an ohmic contact 124, and an n-type semiconductor 125, which is provided with an ohmic contact 126. The two ohmic contacts 124, 126 are optionally connected through a metal contact 127, which serves to act as a support for the side-by-side diode 120, which is shown suspended in reaction matrix 128.
As alternatively shown in cross-section in FIG. 1D, an exemplary stacked p-n junction type photochemical diode 130 is configured to absorb incident light 131 on both sides and includes a p-type semiconductor 133, which is provided with an ohmic contact 134, and an n-type semiconductor 135, which is provided with an ohmic contact 136. The two ohmic contacts 134, 136 are shown connected through an optional metal contact 137.
FIG. 1E illustrates an exemplary apparatus 140 for producing hydrogen and bromine from hydrobromic acid, as disclosed in Meyerand et al. Apparatus 140 includes platelet particles 141, a hydrobromic acid electrolyte solution 144 flowing as indicated by arrows 145. The flow of the electrolyte solution 144 can be such that the platelet particles 141 remain substantially suspended between the area defined by lower and upper screens 146a, 146b. Lower screen 146a primarily provides a resting place for the platelets 141 during shutdown, while lower screen 146a and upper screen 146b confine errant platelets 141 during positive and negative flow surges and turbulence. Screens 146a, 146b can be optionally removed once system stability is attained. Radiant energy 143 drives the formation of bromine 147, which settles to the bottom of the apparatus, and hydrogen gas 149, which bubbles to the surface and is expelled through port 148. The hydrobromic acid electrolyte solution 144 can be run through a monitoring station 150 and additional electrolyte added as needed.
Notwithstanding the foregoing, commercially feasible production of hydrogen, bromine and other products using semiconductor powered devices remains elusive and no commercial system for producing chemicals of any kind using artificial solar photoelectrosynthesis has ever been demonstrated. Accordingly, there remains a need to find commercially and technically feasible ways to utilize solar energy to drive useful chemical reactions. There is also a need to produce hydrocarbon fuels from renewable energy sources such as solar energy. There is also an ongoing need to remove or sequester carbon dioxide from the atmosphere.